Two-component polymeric materials such as reactive adhesives, paints, gasket materials, and caulking materials comprise two separate components which react chemically with one another when intermixed. For example, two-component hot melt polymeric materials used in adhesive applications include a polymeric material and a second material such as a hardener. These types of hot melt adhesives, and other two-component polymeric materials, are dispensed from a system in which the two components are supplied in a predetermined ratio to a mixer/dispenser where they are intermixed with one another and dispensed onto a substrate. In such a system, if too much of one component is applied, then the characteristics of the combined materials are undesirably altered. It is therefore important that the ratio of the components of two-component mixing and dispensing systems be exactly maintained. This ratio is particularly difficult to maintain when the materials are supplied to a dispenser which is intermittent in operation, i.e., which is repeatedly turned on and off. In such applications, loss of ratio control characteristically occurs for a few seconds shortly after the dispenser is opened. During that first few seconds after opening of the dispenser, a transient imbalance phenomena occurs caused by the elasticity in the system and the changing hydraulic pressures associated with cycling the dispenser.
Another problem which may occur in intermittent operations is a loss of flow control of the resulting mixture of the two components. It is desirous to control the flow rate of the resulting mixture dispensed to the substrate. However, during the first few seconds after opening of the dispenser, the transient imbalance phenomena described above may result in a loss of control of the flow rate of the mixture. If the mixture is an adhesive, this may result in less adhesive being applied to the substrate which, in turn, may affect the bonding of materials. This loss of flow control can occur separately or in addition to the loss of ratio control. In other words, even if the ratio control is not lost after the opening of the dispenser, the flow control may be lost. Therefore, it is desirous to control both the ratio of the components of mixing and the flow rate of dispensing of the resulting mixture.
Two-component liquid, mixing, and dispensing systems conventionally comprise a source for each component connected through metering pumps to the dispenser. Either immediately before the dispenser or at the dispenser, the two components are combined and mixed. In the steady state flow condition of the system, the volumetric ratio of the two components dispensed from the system is controlled by the metering pumps. The exact ratio, though, may be measured as a function of the pressure of the two materials at the dispenser. This pressure results not only from (1) pressure created by the metering pumps, but also from (2) the rate of flow of the materials between the metering pump and the dispenser, and (3) the hydraulic flow restrictions contained between the metering pump and the dispenser. Since pressure is dependent on flow, it changes as the dispenser is cycled, and the flow path changes. All systems, and particularly those in which the dispenser is connected to the metering pumps by flexible hoses, have some resiliency in the hydraulic system. As a consequence, when the hydraulic pressure changes, the volume of stored material between the dispenser and the metering pump changes. When the valve of the dispenser is subsequently opened, an incorrect ratio condition and/or an incorrect flow rate occurs until the inlet pressure at the dispenser of both components reaches equilibrium or steady state flow pressure. Maintenance of a desired ratio of the two components of a two-component system therefore requires that the pressure of each component of the system at the dispenser be adjusted and controlled, not only during the steady state flow condition of the system, but also during the first few seconds after opening of the dispenser valve.
It has therefore been one objective of this invention to provide a two-component mixing and dispensing system which maintains a volumetric or mass relationship between the two components when the system is operated intermittently, both during start-up of flow and during the steady state flow condition.
Another objective of this invention has been to provide a two-component dispensing system which compensates for or eliminates flow and ratio transients which customarily occur in an intermittently operated, two-component mixing and dispensing system immediately after opening of the dispensing valve(s) of the system.
But precise maintenance of a desired ratio of the two components of a two-component mixing and dispensing system requires more than that the pressure of each liquid component at the dispenser be adjusted and controlled. Specifically, it further requires that the ratio of the two components be exactly maintained. In the case of polymeric materials which chemically react with one another, this ratio is determined by the weight or mass of the two component materials. But the two-component materials are generally supplied to the dispenser by volumetric metering pumps, and those volumetric metering pumps, in the absence of appropriate controls, control the volumetric ratio of the two components, rather than the weight or mass ratio. The volumetric ratio fails to account for any changes in density and resulting changes in mass such as occurs whenever there is a temperature change of the materials. Thus, in a volumetric controlled system, changes in temperature of the individual components being mixed, introduces an error in the relative mass ratio between the two components. Otherwise expressed, if the system maintains a fixed volume ratio of two components and the density or specific gravity of one or both components changes, an error is introduced into the weight or mass ratio of the two components. And it is the weight or mass ratio which must be maintained in a two-component mixing and dispensing system wherein the two components chemically react with one another when combined.
It has, therefore, been another objective of this invention to provide a two-component mixing and dispensing system which maintains a fixed ratio of mass of the two components even if and during density or specific gravity changes of one or both of the components.
Density changes of polymeric materials of the type with which this invention is primarily concerned, generally result from temperature changes. It has, therefore, been another objective of this invention to provide a two-component mixing and dispensing system which compensates for mass or weight per unit of volume changes which result from temperature changes in one or both of the components and is operable to maintain a fixed mass ratio of the two components in spite of and during any such temperature and resulting density changes.
Another parameter which must generally be accurately controlled and maintained in a two-component mixing and dispensing system is the volumetric output of the combined component materials dispensed from the dispenser. This output may be expressed as a volumetric output flow rate or as a mass output flow rate. But in either event, whether expressed as a constant volumetric output flow rate or a mass output flow rate, it must, in most applications, be maintained constant. It has, therefore, been another objective of this invention to provide a two-component mixing and dispensing system wherein the volumetric output flow rate from the dispenser or the mass output flow rate from the dispenser is maintained constant while simultaneously the mass ratio of components supplied to the dispenser is maintained constant.
The "mass ratio" of one component relative to another component of a two-component mixing and dispensing system is the same ratio as the mass flow rate to the dispenser of one component relative to the mass flow rate to the dispenser of the other component. As the mass flow rate of one component to the dispenser changes, relative to the mass flow rate of the other component to the dispenser, so does the "mass ratio" of the two components change in the same ratio. Otherwise expressed, the "mass ratio" of two components supplied to a mixing and dispensing system is the same ratio as the mass flow rate of that one component relative to the mass flow rate of the other component.